Anionic layered mixed double hydroxide compositions have a structure similar to the mineral brucite in which divalent ions are octahedrally surrounded by hydroxide anions and the resulting octahedra share edges to form sheets. The sheets, which are one of the identifying characteristics of clays, contain metal cations and hydroxide anions in the ratio of about one to two. Wherever trivalent cations take the place of divalent cations in the sheets, a positive charge results which is balanced by the negative charge of interstitial anions to produce a stable structure. Interstitial anions hold the sheets apart, thereby establishing interstitial layers which are another of the identifying characteristics of clays. For example, in the anionic layered mixed double hydroxide composition known as hydrotalcite, carbonate ions act as interstitial ions to balance positively charged sheets containing divalent magnesium ions, trivalent aluminum ions and negative hydroxide ions. Layered compositions lacking the mixed double hydroxide structure generally perform less favorably as catalysts, or as precursors of catalysts.
In addition to naturally-occurring hydrotalcite, layered mixed double hydroxide compositions have been prepared synthetically. For example, U.S. Pat. No. 3,879,525, in the name of Miyata et al. describes composite metal hydroxides having layered structures which are prepared from water soluble inorganic salts of divalent metals. The compositions are said to be useful for catalytic purposes, absorbents, desiccants and the like.
Preparation of hydrotalcites is described in U.S. Pat. No. 3,539,306 in the name of Kumura et al. Their process comprises mixing an aluminum component such as an aluminum salt, with a magnesium component such as magnesium salt, in an aqueous medium in the presence of carbonate ions. In the patent Kumura et al. state that the aluminum component may be any member of the group consisting of aluminum hydroxide, basic aluminum carbonate, aluminum hydroxide-alkalicarbonate complex, aluminum amino acid salt, aluminum alcoholate, water-soluble aluminum salt and water-soluble aluminate. As the magnesium component, Kumura et al. recommend any member of the group consisting of magnesium oxide, magnesium hydroxide, magnesium carbonate and water-soluble magnesium salt. However, Kumura et al. report no distinction between salt-containing reactants and salt-free reactants.
U.S. Pat. No. 4,539,195 in the name of Schanz et al. describes a process for producing a basic magnesium aluminum hydroxycarbonate of the formula EQU Al.sub.2 Mg.sub.6 (OH).sub.12 (CO.sub.3).sub.3.X H.sub.2 O
which includes conversion of aluminum hydroxide with basic magnesium carbonate and at least one other compound selected from magnesium hydroxide and magnesium oxide. The conversion is described as taking place at temperatures from about 50.degree. C. to about 100.degree. C., after which the resulting product purportedly can be spray dried without any intermediate filtration and washing processes. Schanz et al. state that the X-ray diffraction spectrum for the aluminum magnesium hydroxycarbonate is distinguishable from the spectrum generally associated with hydrotalcite. Significantly, the formula provided by Schanz et al. is not that of a layered mixed double hydroxide composition.
Preparation of hydrotalcite-type clays with more open galleries, generally known as pillared hydrotalcites, was described in U.S. Pat. No. 4,774,212 in the name of Drezdon. The patent states that anion-pillared hydrotalcite-type clays can be made by combining a magnesium salt, an aluminum salt, and an organic material intended as the inter-layer species in an aqueous solution. Reportedly, hydrotalcite having molybdate ions, tungstate ions, and vanadate ions as anionic pillars were prepared. A related procedure, was described in U.S. Pat. No. 4.843,168 in the name of Drezdon et al.
Traditional processes for manufacturing layered mixed double hydroxide compositions utilize metal salts in solution form as reactants. Such metal salt solutions contain dissociated metal cations which are easily dispersed and, therefore, well suited to formation of sheets in layered mixed double hydroxide compositions. However anions liberated by dissociation of metal salts form, generally, undesirable co-products which degrade the purity of the product compositions and/or necessitate additional separation processing for removal of co-products. Typically, complete removal of co-products requires as much as 100 parts by weight of wash water for each part by weight of finished composition. Washing and subsequent disposing of used wash water is burdensome.
U.S. Pat. No. 4,728,635 in name of Bhattacharyya et al. is directed to processes for production of a calcined alkaline earth, aluminum-containing spinel composition for use as a sulfur oxide removal agent. Their preparation procedures, including a procedure which produces a magnesium-rich, magnesium aluminum-containing spinel composition. In the procedure, an aqueous gel-containing slurry was formed by combining water, formic acid, pseudo-bohemite alumina and magnesia. The slurry was subsequently spray-dried and calcined to produce a relatively salt-free product. However, the proportions of the reactants utilized indicate that the product did not possess a layered structure.
U.S. Pat. No. 5,288,675, in the name of Kim, contemplates a MgO/La.sub.2 O.sub.3 /Al.sub.2 O.sub.3 ternary oxide base wherein the MgO component is present as a micro-crystalline phase which may be detected by X-ray diffraction analysis. The ternary oxide base is said to be useful in combination with ingredients such as ceria and/or vanadia to control sulfur oxide emissions. Kim states that the combination can be prepared by a multi-step process which includes reacting an aged, co-precipitated lanthanum and aluminum hydrous oxide slurry with a magnesium oxide slurry and a sodium hydroxide solution, calcining, impregnating with solutions of cerium and/or vanadium and calcining at a temperature of 450.degree. C. to 700.degree. C.
U.S. Pat. No. 5,426,083 in the name of Bhattacharrya et al. and assigned to the assignee of the present invention describes an absorbent and process for removing sulfur oxides from a gaseous mixture, as well as processes for manufacturing the absorbent. The manufacturing processes described in the patent of Bhattacharrya et al. significantly advance the art and are entirely satisfactory for many purposes. However, the manufacturing processes recite blending divalent and trivalent metal salts as reagents. U.S. Pat. No. 5,426,083 is hereby incorporated by reference in its entirety, and particularly for its teachings regarding the preparation, use, calcination, and collapse of layered anionic mixed hydroxide compositions.
A salt is defined for the present purposes as any substance which spontaneously yields ions, other than hydronium or hydroxide ions, when immersed in water. Typically, salts are obtained by displacing the hydrogen of an acid by a metal. According to the definition, metal oxides, metal oxide hydroxides and metal hydroxides are not salts because these compounds spontaneously yield essentially no ions or, alternatively, yield hydroxide ions when immersed in water. Examples of salts under this definition are metal nitrates, metal chlorides, metal acetates, and metal carbonates.
Metal salts, both divalent and trivalent, are known to give rise to byproducts, such as nitrates, which are regarded as objectionable contaminants in some applications. Such byproducts have long been regarded as unavoidable, as previous practitioners apparently believed that aqueous dissociated metal salts were required to form the well ordered sheets characteristic of high quality clays. As a result, conventional processes for manufacturing layered mixed double hydroxide compositions often include tedious filtration and water washing steps to remove the byproducts. A process for preparing superior absorbents which does not include metal salts as reagents would be welcomed by practitioners who object to the presence of byproducts but who also wish to minimize or dispense with filtration and water washing for byproduct removal.
Although previously known methods are entirely satisfactory for many purposes, a need still exists for an improved manufacturing process which produces layered mixed hydroxide compositions from relatively salt-free reactants. Desirably, improved processes should produce less co-product which tend to degrade purity of product and necessitate disposal. More desirably, improved processes utilize reactants efficiently so that costs for salt-free reactants per unit of product are less than that for traditional processes which utilize salt-containing reactants. Improved manufacture of anionic, hydrotalcite-type pillared clay compositions by relatively salt-free methods which generate less spent wash water is especially welcome.
Synthesis gas, a mixture of carbon monoxide (CO) and molecular hydrogen (H.sub.2), is a valuable industrial feedstock for the manufacture of a variety of useful chemicals. For example, synthesis gas can be used to prepare methanol or acetic acid. Synthesis gas can also be used to prepare higher molecular weight alcohols or aldehydes as well as higher molecular weight hydrocarbons.
Perhaps the most common commercial source of synthesis gas is the steam reforming of coal or a hydrocarbon, particularly natural gas. In the steam reforming process, a mixture of water and the hydrocarbon are contacted at a high temperature, for example, in the range of about 850.degree. C. to about 900.degree. C., and typically in the presence of a catalyst, to form a mixture of hydrogen and carbon monoxide. Using methane as the hydrocarbon, the theoretical stoichiometry for the steam reforming reaction is as follows: EQU CH.sub.4 +H.sub.2 O.fwdarw.&gt;3H.sub.2 +CO.
In prior steam reforming processes, because of the large amount of steam typically necessary to reduce coke formation, the ratio of H.sub.2 to CO produced is typically greater than 3:1 and can be 5:1.
The steam reforming reaction is a highly endothermic reaction, and, as discussed above, it produces a relatively high molar ratio of hydrogen to carbon monoxide when conventional processes are used. In some processes using synthesis gas, the excess hydrogen is not necessary and must be separated from the CO. For example, the manufacture of methanol or acetic acid from synthesis gas requires less than a 3:1 molar ratio of hydrogen to carbon monoxide.
There are other methods for preparing synthesis gas that are more attractive than the steam reforming reaction. Synthesis gas produced by the partial oxidation of methane, for example, is an exothermic reaction and produces synthesis gas having a lower ratio of hydrogen to carbon monoxide, according to the following equation: EQU CH.sub.4 +1/2O.sub.2 .fwdarw.2H.sub.2 +CO.
Synthesis gas can also be produced by the reaction of a hydrocarbyl compound such as methane with carbon dioxide. This reaction proceeds according to the following equation: EQU CH.sub.4 +CO.sub.2 .fwdarw.&gt;2H.sub.2 +2CO.
This reaction, like the steam reforming reaction, is endothermic; however, it produces a low ratio of hydrogen to carbon monoxide (1:1) and is very useful where there is an abundant supply of carbon dioxide, for example, at a refinery or near naturally-occurring CO.sub.2 reserves. Additionally, the reforming reaction using carbon dioxide can also be used in conjunction with the steam reforming reaction to adjust the ratio of hydrogen to carbon monoxide.
In all of the hereinabove described processes for preparing synthesis gas, it is advantageous to conduct the reaction of the hydrocarbyl compound with the source of oxygen in the presence of a catalyst. For example, catalysts for the steam reforming of methane and other hydrocarbons are commonly based on nickel as the active catalyst component. Vernon et al. in Catalysis Letters, Vol. 6, pages 181-186, 1990, discloses that methane can be converted to synthesis gas over catalysts such as palladium, platinum, or ruthenium on alumina, nickel on alumina, and certain transition metal oxides including Pr.sub.2 Ru.sub.2 O.sub.7 and Eu.sub.2 lr.sub.2 O.sub.7. While Vernon et al. discloses that nickel on alumina catalysts are effective for the conversion of methane to synthesis gas using molecular oxygen, we have determined that such a catalyst, as well as commercial nickel-containing steam reforming and steam cracking catalysts, form coke and deactivate relatively rapidly. While the other catalysts described in Vernon et al., such as ruthenium on alumina, can be used to reform a hydrocarbyl compound such as methane in the presence of molecular oxygen, such transition metals are expensive and the transition metal catalyst based on ruthenium we evaluated exhibited lower conversion and lower selectivity to synthesis gas compared to the catalysts of this invention. Ashcroft et al. in Nature, Volume 352, page 225, 1991, describes the reforming of methane with carbon dioxide to form synthesis gas using catalysts such as palladium, ruthenium and iridium on alumina, as well as nickel on alumina.
U.S. Pat. No. 3,791,993 to Rostrup-Nielsen discloses the preparation of catalysts containing nickel for reforming gaseous or vaporizable liquid hydrocarbons using steam, carbon oxide, oxygen and/or air. Catalysts disclosed therein are prepared by coprecipitating a nickel salt, a magnesium salt and an aluminate to form a sludge, washing the sludge until it is substantially free of sodium and potassium, drying, and then dehydrating at 300.degree. C. to 750.degree. C. The ultimate catalyst is formed after a calcination step at 850.degree. C. to 1100.degree. C. Examples in Rostrup-Nielsen show that compositions having a 1:1:2 or a 2:7:1 mole ratio of nickel, magnesium and aluminum, respectively, are suitable for converting naphtha to hydrogen-rich gaseous products using steam reforming.
In view of great commercial interest in preparing synthesis gas by reforming readily available hydrocarbon feedstocks such as natural gas, and because of the benefits of conducting these reforming reactions in the presence of a catalyst that remains active for an extended period of use, there is a continuing need for new, less expensive, durable, coke resistant, more active and selective catalysts for the production of synthesis gas. The present invention provides such catalysts as well as a method for preparing synthesis gas using such catalysts.
Catalysts useful in the process of this invention can be prepared from a nickel-containing catalyst precursor compound having a structure that is referred to as "hydrotalcite-like" and/or "hydrotalcite-like." Hydrotalcite compounds are anionic clays, both natural and synthetic, that have a layered or sheet-like structure. For example, hydrotalcite, a naturally occurring mineral, has the chemical composition Mg.sub.6 Al.sub.2 (OH).sub.16 !CO.sub.3.4H.sub.2 O, and is composed of molecular "sheets", each sheet comprising a mixture of magnesium and aluminum hydroxides. The sheets are separated by carbonate ions which balance the net positive charge of the sheets. In these "sheets," the magnesium and aluminum ions are 6-fold coordinate in hydroxide, and the resulting octahedra share edges to form infinite sheets. Water molecules, like the carbonate ions, are randomly located in the space between these sheets. Although pure hydrotalcite contains only magnesium and aluminum cations, a variety of naturally occurring, as well as synthetic hydrotalcite-like compositions are known. A general formula for these hydrotalcite-type compounds is: EQU M.sup.2+.sub.(1-x) M.sup.3+.sub.x (OH).sub.2 !.sup.x+ (A.sub.x/n.sup.n-).mH.sub.2 O,
wherein x generally is a number between 0.1 and 0.50, M.sup.2+ is a 2+ metal ion, for example, Mg.sup.2+ and M.sup.3+ is a 3+ metal ion, for example, Al.sup.3+. The anion, A.sup.n-, can be one of a number of anions such as carbonate. Hydrotalcite-like compounds containing borate as the anion have been disclosed by Bhattacharyya et al., in Inorganic Chemistry, Volume 31, page 3869, 1992. Drezdzon, in Inorganic Chemistry, Volume 27, page 4628, 1988, discloses the synthesis of isopolymetalate-pillared hydrotalcites.
As described above, hydrotalcite-like compounds share the "sheet-like" structural characteristics, which is conveniently identified using x-ray powder diffraction (XRD) analyses. Hydrotalcite-like materials typically have a d(001) value of at least about 7.8 .ANG.. Based on the size of the anion used, the hydrotalcite-like molecules can have d(001) values up to 15 .ANG.. The d(001) value is indicative of the inter layer spacing present in the hydrotalcite-like materials.
Hydrotalcite-like compounds have been used as catalysts in a variety of applications, such as catalysts for aldol condensation, polymerization of alkene oxides, hydrogenation catalysts, dehydrogenation catalysts, etc., as described in F. Cavani et al., Catalysis Today, Volume 11, pages 173--301, 1991. Cavani et al. discloses that coprecipitated Ni, Al-based catalysts have been recognized as satisfying all the requirements for operation in steam reforming for methane production, and that coprecipitated catalysts calcined at 723.degree. K. (450.degree. C.) and reduced at 723.degree. K. (450.degree. C.) were active in the 673.degree. C. to 923.degree. K.(450.degree. C. to 650.degree. C.) range for steam cracking of naphtha to produce methane.
U.S. Pat. No. 3,865,753 to Broecker et al. discloses the use of a catalyst prepared by calcining Ni.sub.5 MgAl.sub.2 (OH).sub.16 !CO.sub.3.4H.sub.2 O at a temperature in the range of 350.degree. C. to 550.degree. C., and which is subsequently reduced with hydrogen. Such a catalyst was used for the steam cracking of hydrocarbons having 2 to 30 carbon atoms at a temperature in the range of 300.degree. C. to 450.degree. C. to form methane.
Ross et al., J. of Catalysis, Volume 52, pages 280-290, 1978, have examined the reaction of methane with water over a catalyst derived from Ni.sub.6 Al.sub.2 (OH).sub.16 CO.sub.3.4H.sub.2 O at temperatures of 873.degree. K. to 973.degree. K. (600.degree. C. to 700.degree. C.). Kruissink et al., J. Chemical Society, Faraday Trans. I, Volume 77, 649-663, 1981, discusses the thermal treatment of nickel-containing compositions having x-ray patterns characteristic of the hydrotalcite-like minerals; and Hernandez et al., Thermochemica Acta, Volume 81, 311-318, 1984, investigated the thermal decomposition of hydrotalcite-like compounds of formula Ni.sub.1-x)Al.sub.x (OH).sub.2 !.sup.x+ X.sup.n.sub.x/n.mH.sub.2 O where x is in the range of 0.25 to 0.33 and X is carbonate and sulfate. Using X-ray diffraction studies, these researchers identified nickel oxide as the decomposition product at temperatures above 600.degree. C., whereas the corresponding spinel, NiAl.sub.2 O.sub.4, was formed at temperatures higher than 1000.degree. C.
British Patent No. 1,342,020 discloses catalysts having chemical composition Ni.sub.6 Al.sub.2 CO.sub.3 (OH).sub.16.4H.sub.2 O and Ni.sub.3 Mg.sub.3 Al.sub.2 CO.sub.3 (OH).sub.16.4H.sub.2 O and discloses that they have an application as hydrogenation, dealkylation and cracking catalysts. Clause et al, J. of Catalysis, Volume 133, 231-246 (1992) discloses the preparation and analysis of nickel-aluminum mixed oxides obtained by thermal decomposition of hydrotalcite-type precipitates. This paper also discloses that nickel-aluminum mixed oxides resulting from the thermal decomposition of hydrotalcite-like coprecipitates have been studied for steam reforming and methanation reactions.